1. Field of the Invention
This invention relates to actuators based on electrochemical double-layer induced charge injection in materials having very high specific surface areas. Preferred embodiments include actuators for accomplishing mechanical work; the control of thermal, electrical, and fluid transport; and the switching, phase shift, and attenuation of electromagnetic radiation. The actuators range from large actuators to microscopic and nanoscale devices. These actuators are either directly powered by an externally provided electrical energy input or by chemical or photonic processes that generate electrical energy input. The electromechanical actuators can be operated in the reverse direction to convert an input mechanical energy to an output electrical energy.
2. Description of the Background Art
The background art includes various means for fabricating electromechanical actuators that are based on magnetostrictive, electrostrictive, ferroelectric, electrostatic, or shape-memory actuator processes. Each of these actuator processes has a disadvantage that prohibits an important category of applications. For example, the magnetostrictive, electrostrictive, and ferroelectric actuator processes suffer either from low achievable actuator strains (typically less than 0.1%) or a low modulus that limits the work capability per cycle. The need for high magnetic fields for the magnetostrictive actuator processes and large electric fields for the electrostrictive and ferroelectric actuator processes are other significant disadvantages for important applications.
Faradaic conducting polymer electrochemical actuators were proposed about a decade ago [R. H. Baughman et al., in Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics, and Molecular Electronics, eds. J. L. Bredas and R. R. Chance (Kluwer, Dordrecht), pp 559-582 (1990)]. Examples of such devices have been described by E. Smela, O. Inganxc3xa4s, and I. Lundstrxc3x6m, Science 268, 1735 (1995); T. F. Otero and J. M. Sansinena, Adv. Mater. 10, 491 (1998); A. Della Santa, D. De Rossi, and A. Mazzoldi, Smart Mater. Struct. 6, 23 (1997); M. R. Gandhi, P. Murray, G. M. Spinks, and G. G. Wallace, Synthetic Metals 73, 247 (1995); J. D. Madden, P. G. Madden, I. W. Hunter, S. R. Lafontaine, and C. J. Brenan, Proceedingsxe2x80x94Workshop on Working in the Micro-World, IEEE IROS96, Osaka Japan, p. 9-18, November 1996; and K. Kaneto, M. Kaneko, Y. Min, and A. G. MacDiarmid, Synthetic Metals 71, 2211 (1995).
Conventional faradaic conducting polymer electromechanical actuators operate by the diffusion of dopant ions to and from solid electrode elements in response to an applied potential. As a result of such diffusion, ions are either inserted or de-inserted from solid electrode elements. As a result of the volume change produced by such dopant insertion and de-insertion processes, the electrodes change dimension and this dimensional change produces the actuator stroke. These ion insertion and de-insertion processes are balanced by electron injection and removal from opposing electrodes. While this electron injection and removal could conceivably produce dimensional changes, these dimensional changes are much smaller than those due to the ion insertion and de-insertion processes for all devices that have been demonstrated. For example, ions having large volume (such as perchlorate) are inserted by diffusion into the conducting polymer electrode, and structurally change the solid polymer by pushing apart polymer chains to cause a dimensional change of the conducting polymer electrode.
Although there has been major development effort focused on making practical devices in accordance with this conventional technology, critically important problems remain. The major problem is that the required dopant insertion and de-insertion processes (called intercalation and de-intercalation) result in slow device response, short cycle lifetimes, hysteresis (leading to low energy conversion efficiencies), and an actuator response that depends on both rate and device history. Such conducting polymer electromechanical actuators use the large faradaic dimensional changes that result from the electrochemical doping of various conducting polymers, such as polypyrroles, polyanilines, polyalkylthiophenes, and polyarylvinylenes. Depending upon the dopant species and whether or not they include solvating species, dimensional changes of from 10% to 30% are conceptually obtainable. Since these dimensional changes occur in weakly bonded directions, the elastic modulus for these directions is low, which limits actuator performance.
There has been a proposal to make an actuator that uses the smaller dimensional changes that are in fiber or sheet directions (R. H. Baughman, Synthetic Metals 78, 339 (1996)). A further proposal in Synthetic Metals 78 is an electromechanical actuator that operates analogously to non-faradaic supercapacitors, instead of by the insertion and de-insertion of dopant species in a solid polymeric electromechanical electrode. As proposed, such theoretical non-faradaic actuators might use the change in length of a polymer chain, a graphite sheet, a fiber, or a nanofiber that results from a change in charge. However, although design approaches are suggested, the key problem as recognized in Synthetic Metals 78 is construction of a practical non-faradaic actuator having very high surface area without destroying mechanical properties. Without both the high surface area and high mechanical properties, a useable mechanical actuator can not be made. The point is that porosity must be introduced in order to obtain high surface area for an actuator electrode having macroscopic dimensions. Known fabrication methods for constructing an actuator electrode having porosity results in actuator electrodes that have mechanical properties reduced to such an extent as to be non-usable. For example, efforts to make an actuator from the high-surface-area carbon fibers produced by Unitika Ltd. failed, since these fibers could not support a useful load in an actuator environment. Moreover, the construction of a useful actuator requires that the actuator electrodes are highly electronically conducting, and the introduction of high surface area is expected to degrade electrical conductivity. Consequently, no viable way was available to make a non-faradaic actuator that could serve a useful function.
For example, the suggested approach in Synthetic Metals 78 of using carbon nanotubes was unrealizable because the required fabrication and purification technology was unavailable for either the precursor nanotubes or any macroscopic form of such nanotubes, such as nanotube sheets. The synthetic methods available when Synthetic Metals 78 was published produced nanotubes only as one component among extremely high concentrations of many other components (like fullerenes and weakly bonded carbon particles) that would eliminate the possibility of making a useful actuator. Moreover, the available nanotube samples included major concentrations of chiral and zigzag nanotubes that are semiconducting or insulating. The presence of such poorly conducting tubes would be expected to hinder charge injection, thus eliminating the possibility of obtaining useable actuation. The possibility of using actuators based on the aggregates of carbon nanofibers would be unworkable since no useable method was available for making such aggregates (either as films or fibers) that had the. required surface areas, electrical conductivities, mechanical properties, and freedom from massive contamination levels of degradative impurities. Likewise, it was impossible to make nanoscale actuators since no methods were available to make the electrical and mechanical contacts that were needed for a successful device. Also, no method was available for surrounding the nanoscale actuator with required electrolyte without causing a degradation of the structure of the nanoscale actuator.
The possible application of graphite sheets as actuators was conjectured in Synthetic Metals 78. The graphite sheets mentioned were individual sheets of graphite having molecular thicknesses (sub-nanometer). There was no available technology that would result in either the synthesis of large, free-standing sheets having molecular thicknesses or the construction of actuators from either substrate-bound large area sheets, microscopic area sheets, or nanoscale area sheets. Hence, a useable technology was not available for making actuators of any size from single graphite sheets. Likewise, although the possibility of making nano-faradaic actuators from conducting organic polymers was conjectured in Synthetic Metals 78, no method was suggested for obtaining the large gravimetric surface areas and large gravimetric capacitances that would enable the application of these materials in non-faradaic actuators.
Moreover, conventional ferroelectric actuators for converting mechanical energy to electrical energy suffer from low electrical energy densities per cycle, as well as poor efficiencies for low frequency operation (because of a significant inter-electrode leakage current through the ferroelectric). Also, various intercalation-based actuators have been evaluated in the past for actuator application in which the actuator response is a change in electrical conductivity, optical absorption, or surface energy. However, each of these devices suffer from cycle life and cycle rate limitations due to the dopant intercalation that is associated with charge injection. Moreover, there is a need for actuators that are multifunctional, such as a fabric for clothing that can be electrically driven between states allowing different degrees of thermal conductivity and water and air permeability. There is currently no satisfactory technology for making such actuators.
An object of the invention is to provide a mechanical actuator simultaneously having extremely high work capacities (per actuator volume or per actuator weight), high power densities (per actuator volume or per actuator weight), high cycle life, and high force generation capabilities. In addition, low voltage operation and high temperature performance are sought.
A further object is to provide an improved actuator for converting electrical energy to mechanical energy; for mechanical dampening or mechanical stiffness change; for the control of fluid flow; for the control of electrical currents and voltages; for the control of heat transport; and for the switching, phase shift, or attenuation of electromagnetic radiation. Conventional actuators do not use non-faradaic processes and devices based on other types of processes have major deficiencies.
The invention provides actuators for converting electrical energy to an actuator output and alternatively for converting a mechanical energy input to electrical energy. Such actuators can provide high work capacity, high power density, high cycle life and high force generation capability. Also, these actuators can provide low voltage operation and high temperature performance. In addition, actuators of the invention are useful for the control of thermal, electrical and fluid transport and for the switching, phase shift, and attenuation of electromagnetic radiation.
The above noted objects and others are fulfilled by an actuator having:
at least one ionically conducting and electronically insulating electrolyte; and
at least two electrically conducting electrodes separated by said at least one electrolyte, at least one electrode of said at least two electrically conducting electrodes being a porous solid with a skeletal density in gm/cm3 of xcfx81 having an accessible gravimetric surface area of at least 150 xcfx81xe2x88x921 m2/gm and an accessible gravimetric capacitance of at least 5 xcfx81xe2x88x921 F/gm and having pores containing an electrolyte that is ionically conducting,
the at least one electrode undergoing an actuator response that provides, in whole or in part, an actuator output upon non-faradaic charge injection responsive to application of an electrical voltage to said at least two electrically conducting electrodes.
The objects are also fulfilled by an electromechanical actuator having:
at least one ionically conducting and electronically insulating electrolyte;
at least two electrically conducting electrodes separated by said at least one electrolyte, at least one electrode of said at least two electrically conducting electrodes being a porous solid having an accessible gravimetric surface area of at least 150 m2/gm and an accessible gravimetric capacitance of at least 5 F/gm, having pores containing an electrolyte that is ionically conducting and having a mechanical modulus of at least 0.5 GPa,
the at least one electrode undergoing a dimensional change that provides, in whole or in part, an actuator output upon non-faradaic charge injection responsive to application of an electrical voltage to said at least two electrically conducting electrodes.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.